There is a great need for simple miniaturized long-stroke actuators with a high force capacity. The actuators should preferably be possible to drive for several days with a portable voltage source and in several cases the positioning accuracy has to be high. There are at most a few, if any, actuator types that come close to these demands and the few suggestions that might fulfil more than a few of these demands are far too complex and costly to be commercially feasible.
The request of low power consumption demands a well-designed component. There exist only two major low-power long-stroke (more than some 100% strain) actuator types in prior art. One is the traditional electromagnetic motor, e.g. a miniature DC motor, and the other is the mechanically resonant ultrasonic motor. The largest drawback with the DC motor is that it is not very suitable for miniaturization. Consequently, the price for a DC motor will become high when miniaturized. Furthermore, the delivered speed and momentum of a miniaturized DC motor are also not perfectly matched with general requirements in small applications and the production cost increases strongly when e.g. small gearboxes are needed.
The general resonant ultrasonic motor reaches a high efficiency, however, typically within a small frequency interval. Several factors such as quality of drive electronics, temperature and wear will furthermore make the properties of the motor deteriorate or at least change in an uncontrolled manner. The frequency interval depends intimately on the sizes and shapes of the driving elements. When using small driving elements, the operation frequency will generally be extremely high and any tolerances will influence the operation even more. Furthermore, linear motion is difficult to achieve by resonant motors, since the wave propagation conditions at the ends of the elements/body to be moved are very difficult to control or predict. Normally a mechanical resonance operates well for rotating motors, while the reflected wave at the ends of a linear motor makes this a less attractive solution. Therefore, there are no examples of prior-art miniaturized linear ultrasonic motors with the desired performance, particularly when the drive electronics has to be a part of the portable device.
The use of electromechanically active materials has previously been demonstrated to yield high forces in relation to the actuator volume, see e.g. M. Bexell and S. Johansson, “Fabrication and evaluation of a piezoelectric miniature motor”, Sensors & Actuators A75 (1999), pp. 8–16. Electromechanically active materials should therefore be interesting candidates for basing miniaturized motors on. One of the main problems is, however, to find a motion mechanism that gives a good performance in terms of efficiency and force at the same time as the construction is robust and the actuator can be manufactured in a simple way.
A preferred approach for providing a miniature motor based on electro-mechanical actuators is to carefully design the actuator so that there is a good matching between the maximum force desired and the force capacity of the material itself. The first step is typically the choice of actuator material. The higher mechanical energy density that can be reached in the material the easier will the following mechanical design be. If the energy density is intermediate it will furthermore be important to use a material with a relatively low internal energy loss per excitation cycle.
The problem with mechanical resonance, discussed above, makes it less desirable to use this phenomenon in small linear motors. Instead, one may turn to use electrical resonances. Naturally, there will always be some applications where a mechanical resonance can be used as an additional method to increase the energy efficiency.
The next step in design is to couple the mechanical deformation of the material with the motion of the component to be moved in such a way that a repetition of the actuator material deformation will add to the movement of the rotor (or corresponding linear moving component). There are several different prior-art mechanisms that have been explored for step repetition, e.g. stepping mechanisms (U.S. Pat. No. 3,902,084), walking movements (U.S. Pat. No. 6,337,532), elliptical vibrations (U.S. Pat. No. 6,437,485), and travelling wave mechanisms (U.S. Pat. No. 4,504,760).
Stepping mechanisms and walking movements are indeed very useful in fine positioning. However, there are some difficulties in achieving a high energy efficiency. Furthermore, motors in prior art have been optimized for positioning accuracy. With the known design possibilities, the stiffness will in such cases be very high. This gives a poor matching with the load and hence a reduced efficiency.
Motors based on elliptical vibrations can be designed with a higher degree of energy efficiency. Instead, the fine positioning becomes more delicate and it might be difficult to get design that is mechanically robust enough for demanding applications.
The travelling wave mechanisms utilize the phenomenon that while a mechanical wave travels in one direction, the top points of the waves, which are the points in contact with the rotor, move in the opposite direction to the wave propagation. The pressure/load orthogonal to the movement direction has to be small or moderate to keep the rotor away from the valleys of the waves. Too high load attenuates or cancels the wave mechanism. To get high speed the rotor should be in contact with only a small fraction of the surface area.